The grease is packed inside rolling bearings to be incorporated in various electric auxiliary machines, industrial equipments, and the like to impart lubricity thereto. The grease is obtained by kneading a base oil, a thickener, and additives to be added to the base oil as necessary. As the base oil, a synthetic lubricating oil such as a mineral oil, an ester oil, a silicone oil, and an ether oil is generally used. As the thickener, a metal soap such as lithium soap and urea-based compounds are generally used. As the additive, an antioxidant, a rust proof agent, a metal deactivator, a viscosity index improver, and the like are added to the base oil as necessary.
With high performance of the home electric appliances, industrial equipments, and the electric auxiliary machines developed in recent years, rolling bearings for use in motors thereof are used in conditions where they are rotated at a high speed at a high temperature. Thus the rolling bearings are demanded to be excellent in durability at high temperatures and high speeds. The rolling bearings are also demanded to have quietness.
As grease, to be packed inside a bearing, which displays a good lubricity in a wide temperature range from a low temperature to a high temperature, restrains the bearing from generating an abnormal noise when it is cold, and is excellent in durability at a high temperature for a long time, the grease composed of the base oil consisting of the synthetic hydrocarbon oil and the predetermined ester oil and the urea-based thickener added to the base oil is known (see patent document 1). As grease which is used for a bearing for a pulley of a car to restrain the generation of brittleness-caused peeling from occurring on the raceway surface of the bearing and the surface of a ball and the generation of an abnormal noise when it is cold, the grease composed of the base oil, consisting of the synthetic hydrocarbon oil and the ester oil, which has the predetermined viscosity and the alicyclic diurea compound, serving as the thickener, which is added to the base oil is known (see patent document 2). As grease for allowing a bearing to have a life before the bearing is seized in the high-temperature and high-speed rotation condition, the grease composed of the base oil containing the ester oil and 3 to 30 wt % of the predetermined diurea compound as its thickener is known (see patent document 3).
As shown in the patent documents 1 through 3, as the grease excellent in its durability at a high temperature, the greases in which the diurea compound is used as the thickener are known. The diurea compounds used in these patent documents are obtained by a reaction between diphenylmethane diisocyanate and monoamine.
But even the lubricating compositions, shown in these patent documents, which are composed of the base oil containing the ester oil and the synthetic hydrocarbon oil and the diurea compound used as the thickener thereof have a problem that the lubricating compositions do not necessarily satisfy the performance demanded in recent years for the bearing for use in the home electric appliances, the industrial equipments, and the like such as the durability thereof in the high-temperature and high-speed rotation condition when the lubricating compositions are packed in the bearing.
In addition, as lubricating grease for use in a high temperature and a high load, the lubricating grease containing the compound, serving as the thickener thereof, which has two cyclic imide bonds essentially present in one molecule and two urea bonds, urethane bonds or amide bonds is known (see patent document 4).
It is essential for the compound containing the imide bond to have the combination of the imide bond with the urea bond, the urethane bond or the amide bond. The heat resistance of the urea bond, the urethane bond, and the amide bond is inferior to that of the imide bond as understood from the comparison between the polyimide resin and polyurethane resin or polyamide resin. Therefore even the thickener described in the patent document 4 has a problem that it is incapable of satisfying the durability in the high-temperature and high-speed condition demanded in recent years.